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A growing number of recent studies have demonstrated the substantial impact of the alkyl side chains on the device performance of organic semiconductors. However, detailed investigation of the effect of side-chain engineering on the blend morphology and performance of ternary organic solar cells (OSCs) has not yet been undertaken. In this study, the performance of ternary OSCs is investigated in a given poly(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)):[6,6]-phenyl-C-butyric acid methyl ester (PTB7-Th:PCBM) host set by introducing various small molecule donors (SMDs) with different terminal side-chain lengths. As expected, the performance of binary OSCs with SMDs depends greatly on the side-chain length. In contrast, it is observed that all SMD-based ternary OSCs exhibit almost identical and high power-conversion efficiencies of 12.0–12.2%. This minor performance variation is attributed to good molecular compatibility between the two donor components, as evidenced by in-depth electrical and morphological investigations. These results highlight that the alloy-like structure formed due to the high compatibility of the donor molecules has a more significant effect on the overall performance than the side-chain length, offering a new guideline for pairing donor components for achieving high-performance ternary OSCs.